1. Field of the Invention
The present invention relates to a sensor control apparatus and a sensor control method for controlling the energization state of a sensor, such as a gas sensor including at least one cell which includes a solid electrolyte body and a pair of electrodes provided on the solid electrolyte body.
2. Description of the Related Art
An oxygen sensor, which detects the concentration of oxygen within an exhaust gas, is known as a gas sensor used for improving the fuel consumption of an internal combustion engine of an automobile or the like and/or for performing combustion control for the engine. Further, there has been a demand for reducing the amount of nitrogen oxides (NOX) within exhaust gas in order to cope with stricter automobile exhaust gas regulations, and NOX sensors which can directly measure NOX concentration have been developed.
Such a gas sensor includes a gas sensor element having one or more cells each composed of oxygen-ion conductive solid electrolyte layer formed of zirconia or the like, and a pair of electrodes formed on the surface of the solid electrolyte layer. The gas sensor is configured to detect the concentration of a specific gas based on the output from the gas sensor element.
One known example of such a gas sensor is a full-range air-fuel-ratio sensor (hereinafter also referred to as a “UEGO sensor”), which is configured such that two cells (an oxygen-concentration detection cell and an oxygen pump cell) are arranged with a measurement chamber disposed therebetween, and a gas to be measured (hereinafter referred to as an “object gas”) is introduced into the measurement chamber via a diffusion resistance so as to detect oxygen contained in the object gas. Further, another known example of such a gas sensor is an NOX gas sensor which includes three cells in total; i.e., the above-described two cells (an oxygen-concentration detection cell and an oxygen pump cell), and a cell for detecting NOX gas concentration.
A sensor drive circuit is connected to such a gas sensor so as to supply electric current to the sensor cells via the sensor drive circuit, and the concentration of a specific gas contained in the object gas is measured based on the output of a sensor cell. A control apparatus, including the sensor drive circuit, is called a “gas sensor control apparatus.” Further, there are various states in which electric current is supplied to sensor cells (hereinafter referred to “energization states”), including an energization state for protecting the gas sensor; a pre-activation energization state in which a minute current is supplied to the gas sensor in a non-activated state; and an energization state for measuring the concentration of a specific gas.
Of these energization states, the energization state for protecting the gas sensor is established in an operation mode in which electric continuity between the sensor cells and the sensor drive circuit is cut off so as to stop the flow of current to the gas sensor and thereby protect the gas sensor. Further, the pre-activation energization state is established in an operation mode in which a minute current is supplied to the oxygen-concentration detection cell. As a result, oxygen of a reference concentration accumulates in a reference oxygen chamber of the oxygen-concentration detection cell, to thereby provide a reference for gas concentration measurement.
Incidentally, wiring lines (electric current supply lines) of the gas sensor or the sensor drive circuit may suffer a wiring anomaly, such as wire breakage or short circuiting with a battery or ground. When, irrespective of the occurrence of such a wiring anomaly, the energization state for gas concentration measurement is continued so as to measure the concentration of the specific gas, an excessively large current flows through the gas sensor, and the gas sensor (cells) may break.
In view of the above, a technique has been developed for cutting off the electrical connection between the sensor drive circuit and the gas sensor upon detecting a wiring anomaly so as to bring the gas sensor into a protected energization state, and then diagnosing the wiring anomaly so as to determine the details and location of the anomaly (see Patent Document 1). This technique prevents excessive current from continuously flowing to the gas sensor, to thereby prevent breakage of the gas sensor.
Further, when an instruction switching the energization state for gas concentration measurement from another energization state is output to the sensor drive circuit without awareness of a wiring anomaly, similarly, an excessively large current flows to the gas sensor, and the gas sensor may break.
In view of the above, a technique has also been developed which permits switching to the energization state for gas concentration measurement only when the state immediately before switching is the pre-activation energization state, to thereby allow for detection of an anomaly without breaking the gas sensor (see Patent Document 2). Even in the case where a wiring anomaly has occurred, if such a wiring anomaly is detected in the pre-activation energization state, in which a minute current is supplied to the gas sensor, the wiring anomaly can be detected. This is because a voltage generated in the gas sensor deviates from a normal range, without excessive current flowing through the gas sensor.
[Patent Document 1] Japanese Patent No. 3833687
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2008-70194
3. Problems to be Solved by the Invention
A conventional gas sensor control apparatus is configured such that, even when a wiring (electric current supply line) anomaly is detected only once, the gas sensor control apparatus immediately determines that an anomaly has occurred, and stops operation of the sensor or switches the energization state of the sensor to an energization state for protection. However, the gas sensor control apparatus may erroneously detect an anomaly, despite the wiring being normal. Further, in some cases, a wiring anomaly such as wire breakage occurs temporarily, and the wiring anomaly suddenly disappears. Therefore, if operation of the sensor is stopped every time an anomaly is detected, wasteful time is needed to return to a gas-concentration-measurement operation. In particular, erroneous detection of a wiring anomaly tends to occur in association with short circuiting.
Further, in the case where the conventional gas sensor control apparatus is modified so as to check whether or not a wiring anomaly is detected through erroneous detection, a new energization state and an additional circuit must be provided. In such a case, the sensor drive circuit has a problem in that it requires additional circuit components, etc., whereby the size of the circuit increases.